The present invention relates to a liquid crystal display, and in particular, to a transflective liquid crystal display.
Display devices are media to visually transfer information to humans and are important for humans and their society in the highly developed information environment today. Liquid crystal displays have been tremendously developed in performance and hence are adopted as displays for various apparatuses such as cellular phones, personal computers, and large-screen television sets. In general, a liquid crystal display includes a liquid crystal display panel and a backlight (illuminating device) arranged on the rear surface of the display panel to emit light onto the panel. To display a color image, each pixel includes three subpixels respectively corresponding to, for example, three primary colors, i.e., red, blue, and green. The subpixel of each color can be independently controlled to reproduce various colors.
For mobile devices such as cellulars, a transflective liquid crystal displays is employed because of its performance to cope with wide-range illumination environments. In a display area of a liquid crystal panel of such transflective liquid crystal display, each subpixel includes a transmission area and a reflection area. The transmission area achieves transmissive display by controlling the amount of transmitted light from the backlight. On the other hand, the reflection area implements reflective display by controlling the amount of reflected light from the external area. That is, the transflective liquid crystal display primarily conducts transmissive display in a dark environment and reflective display in a bright environment to guarantee visibility of the display image, and hence is usable in a wide-range illumination environment.
In this connection, a liquid crystal display of In Plane Switching (IPS) type is known as a liquid crystal display having a wide viewing angle. The liquid crystal display of IPS type includes, like liquid crystal devices of other types such as a Vertical Alignment (AV) type and a Twisted Nematic type, a pair of transparent substrates, a liquid crystal layer intercalated therebetween, a pair of polarizers disposed respectively on surfaces of the associated transparent substrates, the surfaces of the substrates respectively opposing to the surfaces facing the liquid crystal layer. In the display, the state of polarization of light incident to the liquid crystal layer is changed to control the amount of transmitted light to thereby display an image. In the display of IPS type, pixel electrodes to drive the liquid crystal and common electrodes are formed on one and the same substrate. By providing a voltage potential difference between these electrodes, movement of liquid crystal molecules is controlled. The amount of transmitted light is controlled mainly by the movement of liquid crystal molecules in which the molecules rotate in a plane parallel to the associated plane of the substrates. This leads to a wide viewing angle for the following reasons. That is, when the screen of the display is viewed from an inclined direction, there do not occur disadvantages in which the contrast ratio of the displayed image abruptly lowers and inversion takes place in gradation. Transmissive liquid crystal displays of IPS type have already been put to practices in various fields. It has been highly expected that transflective liquid crystal displays are introduced to practical uses.
JP-A-2006-184325 describes a transflective liquid crystal display of IPS type including a polarizing layer which coats pixel electrodes and common electrodes. Specifically, in addition to the paired polarizers of the liquid crystal device, a new polarizing layer is arranged to coat the pixel electrodes and the common electrodes. The paired polarizers are disposed such that the absorption axes respectively thereof are perpendicular to each other, and the absorption axis of the polarizing layer is aligned with that of the polarizer disposed on the backlight side. In the configuration, in the transmissive display mode, when the driving voltage is zero, black (dark) display results and when the driving voltage is applied, bright display results basically as in the conventional liquid crystal display of IPS type. The display is hence a display of normally black type.
For the reflective display, the pixel electrodes and the common electrodes are formed using a reflective electrode material. Or, a reflecting layer is disposed as a layer other than the layers of these electrodes to provide a reflection area. Therefore, like in the transmissive display, the black (dark) display is implemented when the driving voltage is zero and the bright display is implemented when the driving voltage is effectively applied. It is hence possible to realize a transflective liquid crystal display of normally black type.
In the technique of JP-A-2006-184325, a Thin Crystal Film (TCF) of OPTIVA, Inc. is employed as the material of the polarizing layer. That is, a material of coating type is used for the polarizing layer. In this connection, materials of coating type to be used for the polarizing layer are described in pages 1170 to 1173 of “Current Status and Future Prospect of In-Cell Polarizer Technology” written by Y. Ukai et al. in SID 04 DIGEST, 2004 and in pages 1316 to 1319 of “Ultra-Thin O-Polarizers' Superiority over E-Polarizers for LCDS” written by Ir Gvon Khan et al. in SID 04 DIGEST, 2004.